Production riser with pre-formed curves for accommodating vessel motion

ABSTRACT

A system for establishing a fluid communication between a floating-body and a wellhead is provided. The system comprises a means for fluidly connecting the floating-body and the wellhead that defines a major axis essentially extending from about the floating-body to about the wellhead. The system further comprises a means for absorbing and releasing energy in response to the heave and surge of the floating-body that flexes in a direction essentially parallel to the major axis. An apparatus is also provided for establishing a fluid connection between a floating-body and a wellhead that is fixed to the seafloor. The apparatus comprises a riser pipe and a series of pre-formed curves in the riser pipe that defines a substantially linear axis between the floating-body and the wellhead.

BACKGROUND

[0001] This invention is generally related to risers that convey fluid from producing wells on the seafloor to a floating structure on the sea surface. This invention is also related to a conduit that is fixed to the seafloor, which must accommodate the motion of a vessel that is connected to it.

[0002] In offshore drilling and production operations carried out from a floating vessel, fluid is conveyed from wells on the seafloor to the vessel stationed on the surface by a conduit often referred to as a “riser.” Various methods and mechanisms are used to reduce stresses in risers that are affixed to the moving vessel on the surface and the stationary wellhead at the seafloor. These include using flexible hose for the riser in lieu of steel pipe, supporting a steel riser with hydraulic or elastomeric tensioners that accommodate the relative movement of the vessel, buoyancy cans that support the pipe at the top and allow the vessel to move (as shown in, for example, U.S. Pat. No. 4,702,321, incorporated herein by reference) or some combination of these techniques. Another method is by using a steel catenary riser (often referred to as a “SCR”), which comprises an extension of the steel riser pipe a sufficient horizontal distance from the vessel such that the pipe forms a rather deep catenary curve. Depending on a number of factors, the SCR can be designed to accommodate some vessel motion.

[0003] The above methods all have disadvantages and limitations. For example, flexible hose is costly, cannot withstand external compressive loads without internal stiffening, and requires bend-restrictor devices at the terminations. The SCR are much less costly and have a long record of reliability; however, their shortcoming lies in motion compensation. The tensioners and buoyancy cans are expensive, and they both require a flexible hose (referred as a jumper line) to accommodate the relative motion between the top of the riser, which sometimes includes a “Christmas tree,” and a flow manifold (fixed to the vessel).

[0004] There is therefore a need for a relatively low-cost, simple riser that compensates for the motion of a floating vessel.

SUMMARY OF THE INVENTION

[0005] The above issues are addressed by various aspects of the invention using a curved riser.

[0006] A system for establishing a fluid communication between a floating-body and a wellhead is provided. The system comprises a means for fluidly connecting the floating-body and the wellhead that defines a major axis essentially extending from about the floating-body to about the wellhead. The system further comprises a means for absorbing and releasing energy in response to the heave and surge of the floating-body that flexes in a direction essentially parallel to the major axis. In one embodiment, the means for fluidly connecting comprises a steel riser; and, in a typical embodiment, the means for storing and releasing comprises a series of pre-formed curves in the steel riser. In some embodiments, the series of pre-formed curves comprises a series of single-planar, pre-formed curves in the riser. These single-planar curves comprise arcs having a substantially constant radius of curvature in one embodiment and sinusoidal curves in another embodiment. In some other embodiments, the series of pre-formed curves comprises helical pre-formed curves. In various embodiments, the means for storing and releasing comprises pre-formed curves in the steel riser. In some embodiments, these pre-formed curves comprise single-planar curves and in other embodiments, the pre-formed curves comprise helical curves.

[0007] An apparatus for establishing a fluid connection between a floating-body and a wellhead, wherein the wellhead is fixed to the seafloor is also provided. The apparatus comprises a riser pipe and a series of pre-formed curves in the riser pipe that defines a substantially linear axis between the floating-body and the wellhead. In some embodiments the series of pre-formed curves comprises multi-planar curves, which in one embodiment comprises helical shaped curves. In some other embodiments, the series of pre-formed curves comprises single-planar curves, which in one embodiment comprises sinusoidal curves. The riser pipe comprises steel in some embodiments, but in some embodiments, the pre-formed curves comprise a different material than the remainder of the riser pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a side view of an example embodiment of riser with a series of pre-formed curves.

[0009]FIG. 2 is a side view of an example embodiment of a pre-formed helical curve.

[0010]FIG. 3 is a side view of an example embodiment of a pre-formed curve in a single plane.

DESCRIPTION OF EXAMPLE EMBODIMENTS OF INVENTION

[0011] According to one example embodiment of the invention, seen in FIG. 1, various shortcomings of prior designs using a riser pipe 14 are addressed by including pre-formed curves 10 in the riser pipe 14. These curves accommodate the stress generated by the motion of the floating-body 16. The pre-formed curves 10 flex in response to the motion of the floating-body 16, so that the forces generated by the floating-body 16 are not transmitted to the wellhead 12. By using these pre-formed curves 10, a single, steel riser pipe 14 becomes feasible to connect the floating-body with the wellhead 12, without the need for a using catenary curve. In other embodiments, the pipe is made of steel alloy or some other similar alloy. By accommodating the motion of the floating-body 16, the pre-formed curves 10 also eliminate the need for a flexible section of pipe to connect the riser pipe 14 and the floating-body 16. Using a steel riser pipe 14 further eliminates the need for the external stiffening associated with using a flexible pipe section because the steel can withstand the external compressive loads exerted by the environment. In one embodiment, the pre-formed curves 10 are fashioned from a different material than the remainder of the riser pipe 14.

[0012] By absorbing the forces exerted by the floating-body 16 without using a catenary curve, the pre-formed curves 10 also eliminate the need for additional buoyancy devices. As a result, in one embodiment, the riser pipe 14 connecting the wellhead 12 to the floating-body 16 is only suspended from the floating-body 16. In one embodiment, the suspension from the floating-body 16 supports the entire weight of the riser pipe 14, while in another embodiment, part of the riser pipe's 14 weight is supported by the riser pipe 14 itself.

[0013] In some embodiments, the pre-formed curves 10 do not affect the overall orientation or direction of the riser pipe 14. Therefore, in one embodiment, the floating-body 16 suspending the riser pipe 14 is positioned directly above the wellhead 12. The riser pipe 14 thereby defines an axis 21 essentially from about the floating-body 16 to about the wellhead 12. So that the riser pipe 14 can accommodate the motion of the floating-body 16, the pre-formed curves 10 flex in a direction essentially parallel to the axis 21 defined by the riser pipe 14. In another embodiment, positioning the floating-body 16 closer to the wellhead 12 simplifies the installation and design of the subsea systems, in part by enabling a vertical connection between the riser pipe 14 and the wellhead 12. Tools pass more easily through a vertical wellhead 12 connection than through a horizontal connection.

[0014] In one embodiment, the series of pre-formed curves 10 is connected to or close to the wellhead 12 on one end and the remainder of the steel riser pipe 14 on the other end. In still other embodiments, the series of pre-formed curves 10 extends from the wellhead 12 to the floating-body 16. While in still other embodiments, segments of relatively straight riser pipe 14 are on either end of the series of pre-formed curves 10. In the example shown in FIG. 1 the riser pipe 14 connects with a floating-body 16 (in this example, a SPAR-type semi-submersible) and a series of pre-formed curves 10. Other embodiments of the floating-body 16 include a floating production storage and offloading (FPSO) system, semi-submersible platforms, a tension leg platform, and others known to those of ordinary skill in the art. This connection between the wellhead 12 and the floating-body 16 allows fluid to flow from one section to another (sometimes referred to as “fluid communication”). In some examples, this connection also allows tools to be passed from one section to another, and in one specific embodiment, the riser pipe 14 is raised using some lifting means located on the floating-body 16, stretching the series of pre-formed curves 10 and allowing tools to pass more easily through the series of pre-formed curves 10.

[0015] Referring now to FIGS. 2 and 3, examples of pre-formed curves 10 are shown. In FIG. 2, the pre-formed curves 10 comprise an open coil. In one embodiment, this forms a helical curve. As shown in FIG. 2, the vertical distance between equivalent points in the helical curve is called the curve spacing 17, and the curve diameter 18 describes the diameter of the cross-sectional area of the curve. In some embodiments, the curve spacing is at least double that of the curve diameter. In one embodiment, the curve spacing increases as the helical curve rises higher above the seafloor.

[0016] The characteristics of one set of example embodiments of a riser pipe 14 with helical pre-formed curves 10 is shown in Table 1 below. TABLE 1 Riser Pipe Embodiments With Helical Pre-Formed Curves No L A TL OD Wt D/t RF RFr St Kr PS_(ksi) PS_(psf) P_(ksi) * 10² L_(30ksi) 1 30 3.4 240 6.625 0.4321 15.3 24.6 1.0 1.2 1.0 127.1 1.83E+07 183.0 1016.7 2 20 3.4 240 6.625 0.4321 15.3 19.5 0.8 1.0 0.8 85.1 1.23E+07 122.5 680.6 3 30 2 240 6.625 0.4321 15.3 84.6 3.4 4.2 3.4 294.4 4.24E+07 424.0 2355.6 4 30 5 240 6.625 0.4321 15.3 9.2 0.4 0.5 0.4 59.0 8.49E+06 84.9 471.7 5 40 3.4 240 6.625 0.4321 15.3 27.5 1.1 1.4 1.1 153.9 2.22E+07 221.6 1231.1 6 20 3.4 240 6.625 0.2161 30.7 10.8 0.4 0.5 0.4 88.1 1.27E+07 126.8 704.4 7 30 2 240 6.625 0.2161 30.7 46.9 1.9 2.3 1.9 304.9 4.39E+07 439.0 2438.9 8 30 3.4 240 6.625 0.2161 30.7 13.6 0.6 0.7 0.6 131.3 1.89E+07 189.0 1050.0 9 30 5 240 6.625 0.2161 30.5 5.1 0.2 0.3 0.2 61.0 8.79E+06 87.9 488.3 10 40 3.4 240 6.625 0.2161 30.7 15.3 0.6 0.8 0.6 159.0 2.29E+07 229.0 1272.2 11 30 3.4 240 8 0.5229 15.3 52.1 2.1 2.6 2.1 153.6 2.21E+07 221.3 1229.2

[0017] L represents the curve spacing measured in feet.

[0018] A represents the curve radius measured in feet.

[0019] TL represents the total length of the curve used for simulation purposes measured in feet.

[0020] OD represents the outer diameter of the riser pipe measured in inches.

[0021] Wt represents the wall thickness of the riser pipe wall measured in inches.

[0022] D/t represents the ratio of the outer diameter to the thickness of the riser pipe wall.

[0023] RF represents the reaction force necessary to displace the top of the planar sine wave riser 20 feet.

[0024] RFr represents a normalization of the reaction forces to a base case scenario.

[0025] K represents the stiffness of the riser model.

[0026] Kr represents a normalization of the stiffness to a base case scenario.

[0027] PS_(ksi) represents the peak stress in kips per square inch.

[0028] PS_(psf) represents the peak stress in pounds per square foot.

[0029] PS_(ksf*100) represents the peak stress in kips per square foot multiplied by 100.

[0030] L_(30ksi) represents the length of a curved section with necessary to maintain a maximum stress of 30 ksi in the riser.

[0031]FIG. 3 shows an example embodiment in which the series of pre-formed curves 10 comprises curves in a single plane. In some embodiments, these single-planar, pre-formed curves 10 are sinusoidal; and, in other embodiments, the pre-formed curves 10 have semi-circular or other shapes. Combinations of such shapes of varying complexity are included in still further example embodiments. In one embodiment, the pre-formed curves 10 comprise several connected segments of pipes. As shown in FIG. 3, the vertical distance between equivalent points in the sinusoidal curve is called the wavelength 19, and the amplitude 20 describes the width of the curve.

[0032] The characteristics of one set of example embodiments of a sinusoidal riser pipe 14 is shown in Table 2 below. TABLE 2 Riser Pipe Embodiments With Sinusoidal Pre-Formed Curves No L A TL OD Wt D/t RF RFr St Kr PS_(ksi) PS_(psf) P_(ksf)* 10² L_(30ksi) 1 30 2.0 210 6.625 0.4321 15.3 425.4 2.8 21.3 2.8 815.5 1.17E+08 1174.3 5708 2 30 3.4 210 6.625 0.4321 15.3 150.4 1.0 7.5 1.0 461.9 6.65E+07 665.1 3233 3 20 3.4 220 6.625 0.4321 15.3 135.5 0.9 6.8 0.9 411.0 5.92E+07 591.8 3014 4 40 3.4 200 6.625 0.4321 15.3 85.8 0.6 4.3 0.6 508.8 7.33E+07 732.7 3392 5 30 3.4 420 6.625 0.4321 15.3 72.1 0.5 3.6 0.5 228.3 3.29E+07 328.7 3196 6 30 5.0 210 6.625 0.4321 15.3 63.2 0.4 3.2 0.4 288.7 4.16E+07 415.8 2021 7 30 2.0 210 6.625 0.2161 30.7 235.4 1.6 11.8 1.6 844.3 1.22E+08 1215.8 5910 8 30 3.4 220 6.625 0.2161 30.7 75.0 0.5 3.7 0.5 425.4 6.13E+07 612.5 3119 9 40 3.4 200 6.625 0.2161 30.7 85.8 0.6 4.3 0.6 508.8 7.33E+07 732.7 3392 10 20 3.4 220 6.625 0.2161 30.7 75.0 0.5 3.7 0.5 425.4 6.13E+07 612.5 3119 11 30 3.4 420 6.625 0.2161 30.7 39.9 0.3 2.0 0.3 236.3 3.40E+07 340.3 3308 12 30 5.0 210 6.625 0.2161 30.7 34.0 0.2 1.7 0.2 298.9 4.30E+07 430.4 2092 13 30 3.4 210 8 0.5229 15.3 318.8 2.1 15.9 2.1 559.0 8.05E+07 805.0 3913

[0033] L represents the wavelength measured in feet.

[0034] A represents the amplitude measured in feet.

[0035] TL represents the total length of curves used for simulation purposes measured in feet.

[0036] OD represents the outer diameter of the riser pipe measured in inches.

[0037] Wt represents the wall thickness of the riser pipe wall measured in inches.

[0038] D/t represents the ratio of the outer diameter to the thickness of the riser pipe wall.

[0039] RF represents the reaction force necessary to displace the top of the planar sine wave riser 20 feet.

[0040] RFr represents a normalization of the reaction forces to a base case scenario.

[0041] K represents the stiffness of the riser model.

[0042] Kr represents a normalization of the stiffness to a base case scenario.

[0043] PS_(ksi) represents the peak stress in kips per square inch.

[0044] PS_(psf) represents the peak stress in pounds per square foot.

[0045] PS_(ksf*100) represents the peak stress in kips per square foot multiplied by 100.

[0046] L_(30ksi) represents the length of a curved section with necessary to maintain a maximum stress of 30 ksi in the riser.

[0047] One important benefit derived from including pre-formed curves 10 is that they add an additional layer of safety for the structural integrity of the whole riser pipe 14. If, for example, the top end of the riser pipe 14 should move beyond its normal operating design limits either horizontally or vertically, the pre-formed curves 10, in various example embodiments, flex, without local buckling, and still maintains structural integrity. This situation might occur if, for example, the surface vessel 16 should lose buoyancy due to a damaged tank, if the moorings were to come loose or some other mishap were to occur.

[0048] In addition to the characteristics of a riser pipe 14 with pre-formed curves 10 shown in the tables above, a number of additional design factors are considered to develop a site-specific design. A non-exhaustive list of these additional factors includes:

[0049] Water depth

[0050] Envelope of surface vessel motion

[0051] Physical properties of the riser

[0052] Ocean currents

[0053] Envelope of deflection curve of the riser to avoid clashing

[0054] Method of installation and removal of riser

[0055] Limitation of curvature of riser to allow passage of through-tubing tools (e.g. “pigs”).

[0056] The above Summary and Detailed Description are given by way of example only. Other aspects and examples of the invention will be understood to be within the spirit of the present invention and with the scope of or equivalent to that described by the claims. 

I claim:
 1. A system for establishing a fluid communication between a floating-body and a wellhead, the system comprising: a means for fluidly connecting the floating-body and the wellhead, wherein said means for fluidly connecting defines a major axis essentially extending from about the floating-body to about the wellhead; and a means for absorbing and releasing energy in response to the heave and surge of the floating-body, wherein said means for absorbing and releasing energy flexes in a direction essentially parallel to the major axis; wherein the means for fluidly connecting comprises a steel riser; wherein the means for storing and releasing comprises a series of pre-formed curves in the steel riser; and wherein the series of pre-formed curves comprise a series of single-planar, pre-formed curves in the riser.
 2. A system as in claim 1 wherein the series of single-planar curves comprise arcs having a substantially constant radius of curvature.
 3. A system as in claim 1 wherein the series of single-planar curves comprises sinusoidal shaped curves.
 4. A system for establishing a fluid communication between a floating-body and a wellhead, the system comprising: a means for fluidly connecting the floating-body and the wellhead, wherein said means for fluidly connecting defines a major axis essentially extending from about the floating-body to about the wellhead; and a means for absorbing and releasing energy in response to the heave and surge of the floating-body, wherein said means for absorbing and releasing energy flexes in a direction essentially parallel to the major axis; wherein the means for fluidly connecting comprises a steel riser; wherein the means for storing and releasing comprises a series of pre-formed curves in the steel riser; and wherein the series of pre-formed curves comprise a series of multi-planar, pre-formed curves in the riser.
 5. A system as in claim 4 wherein the series of multi-planar curves comprise arcs having a substantially constant radius of curvature.
 6. A system as in claim 4 wherein the series of multi-planar curves comprises sinusoidal shaped curves.
 7. A system as in claim 6 wherein the series of multi-formed curves comprise helical pre-formed curves. 